Integrated circuit devices, such as silicon based semiconductor circuits, are encased within a package to protect the device from mechanical shock. The packages are manufactured from a variety of materials, such as plastics, ceramics and metals.
U.S. Pat. No. 4,939,316 to Mahulikar et al discloses a metallic package. The patent, which is incorporated by reference in its entirety herein, discloses an anodized aluminum alloy base and cover defining a cavity. A leadframe is disposed between the base and cover and bonded to both by an adhesive seal ring. An integrated circuit device is electrically interconnected to the leadframe and encased within the cavity. The assembled package is light weight, durable and efficiently dissipates heat generated by the device.
The aluminum alloy package components have a coefficient of thermal expansion significantly lower than the adhesive sealants, typically an epoxy, used to seal the package. When the packages are heated, thermally generated mechanical stresses can cause delamination between the leadframe and the adhesive. As the packages are heated or cooled, air trapped within the package cavity expands and contracts. The cavity has a fixed volume so the expansion and contraction of the trapped air is compensated as changes in air pressure. The pressure changes apply a stress on the seal ring which could eventually lead to delamination.
Seal ring delamination results in a gross leak failure. One way to identify gross leak failure is by immersing the package in an inert liquid fluorocarbon heated to 125.degree. C. typically for about 1 minute. A stream of bubbles emanating from the package indicates seal delamination and a passageway to the package cavity. Water vapor and other contaminants can enter the package cavity leading to corrosion of the device or electric circuitry.
One method to prevent seal delamination is to increase the width of the seal path. This solution is contrary to a primary electronic package manufacturing objective, namely to maximize the area available within the package cavity for housing the electronic device. One continuing goal of electronic packaging is to encapsulate the largest device possible within a given package outline.
As discussed below, Applicants increase the area available for the semiconductor device by filling the package cavity with a compliant polymer, such as a silicone gel. In U.S. Pat. No. 4,961,106 to Butt et al a package cavity is filled with a fluid or gel having good thermal conductivity, such as helium or hydrogen gas and silicone or silicate gels.
U.S. Pat. No. 5,060,114 to Feinberg et al discloses a conformable gel pad inserted within the cavity of a package. The gel pad has a compressible matrix made from silicone or polyurethane with a thermally conductive material dispersed throughout.
"Glop topping" is utilized in molded plastic packages as disclosed in U.S. Pat. No. 4,965,227 to Chang et al. The package cavity is filled with a soft gel, for example silicone, to provide environmental protection. The glop top is used in molded packages because the molding resin is permeable to moisture. The glop top prevents corrosion of package components by inhibiting moisture diffusion.
While the addition of a compliant polymer to a package cavity has been disclosed for improving thermal conductivity and for providing a moisture barrier, compliant polymers have not, until now, been used to reduce the seal path width thereby increasing the size of the electronic device which can be encapsulated within a fixed package outline.